A growing class of display devices employs phosphor coated segmented anodes excited by low energy thermoelectrons emitted by a dull red filament controlled by a control grid either interposed therebetween or on the outside of the filament. These devices are enclosed in enclosures in which a hard vacuum is drawn. A transparent window in the enclosure enables viewing of the excited anode segments usually through the control grid. The segmented anodes may be in any configuration such as 7-segment numeric or multiple-segment alphabetic as illustrated in U.S. Pat. No. 3,986,760 or they may be in a sequentially illuminated linear indicator such as disclosed in U.S. patent application Ser. No. 732,330.
Vacuum fluorescent display devices are being widely chosen for display purposes because of their brightness, reliability, color and compatibility for drive by solid state descrete or integrated circuits. In the typical application, the value to be displayed is an analog or digital signal which must then be processed to yield one binary signal per anode segment to command each anode segment to be either illuminated or extinguished. More sophisticated systems can, in addition, incrementally control the brightness of the anode segments between full off and full on.
FIG. 1 shows the method used in the prior art to perform the simple function of driving a 7-segment numeric display 10 from a 4-line binary-coded-decimal input 12. An evacuated enclosure 14 of the display 10 is mounted on a suitable support such as a circuit board 16. An integrated circuit 18 in its evacuated enclosure 20 is also mounted on the circuit board 16. One signal line 22 per segment, a total of 7 lines in the illustrated example, is connected between the evacuated enclosure 20 and the evacuated enclosure 14 to connect the 7 binary signals between them. Thus, exclusive of DC power inputs, the integrated circuit evacuated enclosure 20 has 11 hermetically sealed leads 24 piercing it and the display evacuated enclosure 14 has 7 hermetically sealed leads 24 piercing it. Furthermore, the leads 24 are connected to the signal lines 22 by interconnections 26 at each of the two ends of each signal line 22. Thus there are 14 interconnections 26 for the 7 signal lines 22. In the electronics industry, faulty interconnections 22 account for a large part of both assembly labor and device failures. An even more exaggerated example of interconnect requirements is to be found in the linear scale indicator of the referenced U.S. patent application. A 40-segment linear scale indicator as illustrated at 28 in FIG. 2 require 40 signal lines 22, 80 hermetic scals and 80 interconnections to control the 40 segments from a driver integrated circuit 30. In a typical application, only a single analog input signal line 32 is required into the integrated circuit 30. The integrated circuit 30 generates a contiguous set of binary ones on the signal lines 22 to the linear scale indicator 28 in proportion to the amplitude of the analog signal on the signal line 32.